1. Field of the Invention
The present invention relates to heat-resistant film suitable for use as thermal transfer recording medium such asthermal transfer ink ribbons.
2. Description of the Related Art
Ink ribbons for use with thermal dye-sublimation printers and thermal wax printers generally include a heat-resistant film, composed of film substrate, such as polyester film, and a heat-resistant slip layer disposed on one surface of the film substrate, and an ink layer disposed on the other surface of the film substrate. Heat-resistant slip layers are generally produced by dispersing a slip additive in a binder and shaping the binder into a film.
For example, one type of heat-resistant slip layer (1) is shown in Japanese Patent No. 2521885 in which a coating composition is used to form the film that serves as the heat-resistant slip layer. The coating composition contains a material, such as a polyethylene wax or a higher fatty acid amide, that acts as a slip additive or a parting agent when heat-melted in an amount of 10 to 100 parts by weight with respect to 100 parts by weight of a thermoplastic resin binder.
Another type of heat-resistant slip layer (2) shown in Japanese Patent Publication No. Hei 1-49638 uses a different coating composition to form the film that serves as the heat-resistant slip layer. This coating composition is obtained by blending a surfactant in a resin material having a softening point, or melting point, of 100xc2x0 C. or higher to serve as the slip additive. The surfactant is solid or semi-solid at room temperature and has the hydrophilic-lipophilic balance (HLB) of 4 or higher.
Still another type of heat-resistant slip layer (3) proposed in Japanese Patent Laid-Open Publication No. Hei 10-71773 uses another coating composition to form the film that serve as the heat-resistant slip layer. The coating composition is composed of a slip additive, a thermal parting agent and a binder resin. The slip additive is a product produced by the reaction of isocyanate with a straight-chained aliphatic hydrocarbon that includes an amino, a carboxyl, or a mercapt group at one end. The thermal parting agent is a mixture of a metal salt of a higher fatty acid (e.g., calcium salt, magnesium salt, or lithium salt of stearic acid) and a surfactant containing a phosphoric acid ester.
The heat-resistant slip layer (1) disclosed in Japanese Patent No. 2521885 contains the material that acts as a slip additive or a parting agent and melts upon heating in a very large proportion (i.e., 10 to 100 parts by weight) relative to 100 parts by weight of the thermoplastic resin and, thus, the slip additive may bleed onto the surface. The heat-resistant slip layer (2) disclosed in Japanese Patent Publication No. Hei 1-49638 suffers a problem in that the slip additive tends to migrate to an ink layer while the heat-resistant slip layer (3) disclosed in Japanese Patent Laid-Open Publication No. Hei 10-25 71773 has a problem in that the metal salt of a higher fatty acid tends to precipitate or aggregate in the coating composition, reducing the solution stability of the coating composition. As a result, these slip layers may be unsuitable for certain applications.
The recent trend toward fast printing has led to a considerable increase in the amount of energy that a thermal printer head delivers to the heat-resistant slip layer. As a consequence, the heat-resistant film that includes a heat-resistant slip layer such as those described and indicated as (1), (2) and (3) above is more likely to suffer damage including peeling off of the heat-resistant slip layer or deformation due to thermal contraction of the film. Therefore, a need exists for novel heat-resistant film that can resist deterioration, while allowing high volume printing.
In a first aspect of the present invention, a heat-resistant film includes a film substrate and a heat-resistant slip layer disposed on one surface of the film substrate, where the heat-resistant slip layer comprises a binder and a slip additive, and wherein the slip additive is a higher fatty acid metal salt composition comprising a free higher fatty acid in an amount of 3 to 30 wt % and a metal salt of a higher fatty acid.
In a second aspect of the present invention, a heat-resistant film comprises a slip additive, wherein the free higher fatty acid in the slip additive is stearic acid and the metal salt of higher fatty acid is aluminum stearate.
In a third aspect of the present invention, the binder in the heat-resistant film comprises polymethylmethacrylate.
In a fourth aspect of the present invention a heat-resistant slip layer comprises the slip additive in an amount of 3 to 9 parts by weight with respect to 100 parts by weight of a binder.
In a fifth aspect of the present invention, a high glass transition temperature resin layer having a higher glass transition temperature than the binder of the heat-resistant slip layer is interposed between the film substrate and the heat-resistant slip layer.